The present invention is directed to a method of overlaying a plastic film material on a shipping container blank whereby a container subsequently formed from the blank is rendered highly water resistant.
Water resistant shipping containers are required for shipping many types of fresh produce. Containers used to date have principally been prepared by saturating the blank with melted wax before assembly. Other methods have included impregnation with a water resistant synthetic resin or coating the container with a thermoplastic material. In the latter case, forming watertight seals around the edges and any slits or openings has been a major problem. In most cases the resulting containers cannot be recycled and must be disposed of in a landfill.
One early method of plastic coating is seen in Peters, U.S. Pat. No. 3,406,052 where plastic material is applied by extruders depositing a thin film against both faces of a moving box blank. The extruded film is wider than the blank so that it hangs over all edges. These are then sealed with a flame. Alternatively, the edges of the blank may be crushed and sealed with a thermoplastic mixture. No provision was made for sealing any openings other than the edges.
Marshall, in U.S. Pat. No. 3,864,200, describes a box made using resin impregnated linerboards and a two layer corrugated medium having an asphalt barrier coating between the layers. All exposed edges are crushed for a width of about 0.5-1 inch and the asphalt bleeds through the corrugated medium to provide some sealing.
Maughan, in U.S. Pat. No. 3,929,536, shows a moisture resistant corner post usable in packaging materials. This is made from multiple layers of corrugated paperboard fully enclosed in a plastic film. A vacuum former preferably is used to bring the heated shrink wrap plastic film into conformation. End seals are made by an undisclosed method.
Inagaki discloses in U.S. Pat. No. 4,551,123 a tubular paper container sealed within a thermoplastic film. Edges are either plastic wrapped or heat sealed.
Dropsy, in U.S. Pat. No. 4,722,474 shows a package with an internal thermoformed seam free plastic liner. One embodiment also shows a plastic coated outer surface with the outer and inner films being heat sealed along the edges. The plastic coating is apparently applied after formation of the container. French Patent No. 2,645,831 is closely related.
WO 94/02364 describes a container blank formed entirely of a corrugated of thermoplastic material. Edges and all other openings are sealed to add strength and prevent entry of liquid.
WO 90/09927 shows a thermoplastic film encapsulated shipping container blank similar to one made by the process of the present invention. A plastic film is applied to both sides of a corrugated shipping container blank and subsequently heat sealed around all edges and openings. The document is strangely silent in regard to any process steps by which the encapsulated container blank is prepared.
The present invention describes an efficient process by which a plastic encapsulated corrugated shipping container blank may be formed.
The present invention is directed to a method of fully encapsulating corrugated shipping container blanks within a moisture resistant plastic film. The method requires a source of thin, thermobondable, thermoplastic film. This is applied to both sides of a container blank. At a downstream location the film is heated on both sides of the blank to a sufficient temperature so that the film bonds to the blank without additional adhesive. The opposing film layers sag to contact each other along the edges and over the area of any openings and are thermobonded to form seals. After a brief cooling period, excess film along the edges and within any openings is die cut or otherwise removed, leaving the sealed edges intact.
Most preferably, the film is applied as a sleeve wrap in which film supply rolls are located above and below a conveyor system, the axes of the rolls being transverse to the direction of travel on the conveyor. The film from each supply roll spans a planar intermediate space perpendicular to the conveyor travel direction and is thermally bonded end-to end to form a continuous sheet. A container blank moving along the conveyor contacts the film and draws it from the rolls as it travels forward. The film is directed into contact against the upper and lower faces of the container blank by rolls or a similar arrangement. When the container blank has passed the planar intermediate location the film at the trailing edge of the container blank is pinched together and thermally bonded. This provides fore and aft seals of the film wrap. The film at the location of this thermal bond is severed transversely so that the seal remains intact on both sides of the severing line. The wrapped container blank continues down the conveyor line and the film from the supply rolls is again bonded together end-to-end, ready to be engaged by the following container blank. A heating station follows in which the film is bonded to the container blank. At this point the film sags into any slits and other openings and the upper and lower films are bonded together where in contact with each other. Some shrinkage of the film occurs at this time causing the seals to contract tightly against any exposed edges. A die cutter opens the slits and cutouts while keeping the seals intact. The container blank is now fully encapsulated with continuous seals around the peripheral edges and corner slits or other cutout portions and is ready to be formed into a moisture resistant shipping container.
The container when so encapsulated can be readily recycled since the film is easily removed in a repulper and can be readily separated from the fiber portion in conventional fiber cleaning equipment.
It is an object of the invention to provide a method for preparing a highly moisture resistant shipping container.
It is a further object to prepare a moisture resistant shipping container that can be readily recycled.
These and many other objects will become readily apparent upon reading the following detailed description taken in conjunction with the drawings.